A wide variety of semiconductor devices, and methods of making semiconductor devices, are known. Some of these devices are designed to emit light, such as visible or near-visible (e.g. ultraviolet or near infrared) light. Examples include electroluminescent devices such as light emitting diodes (LEDs) and laser diodes, wherein an electrical drive current or similar electrical signal is applied to the device so that it emits light. Another example of a semiconductor device designed to emit light is a re-emitting semiconductor construction (RSC).
Unlike an LED, an RSC does not require an electrical drive current from an external electronic circuit in order to emit light. Instead, the RSC generates electron-hole pairs by absorption of light at a first wavelength λ1 in an active region of the RSC. These electrons and holes then recombine in potential wells in the active region to emit light at a second wavelength λ2 different from the first wavelength λ1, and optionally at still other wavelengths λ3, λ4, and so forth depending on the number of potential wells and their design features. The initiating radiation or “pump light” at the first wavelength λ1 is typically provided by a blue, violet, or ultraviolet emitting LED coupled to the RSC. Exemplary RSC devices, methods of their construction, and related devices and methods can be found in, e.g., U.S. Pat. No. 7,402,831 (Miller et al.), U.S. Patent Application Publications US 2007/0284565 (Leatherdale et al.) and US 2007/0290190 (Haase et al.), PCT Publication WO 2009/048704 (Kelley et al.), and pending U.S. Application Ser. No. 61/075,918, “Semiconductor Light Converting Construction”, filed Jun. 26, 2008, all of which are incorporated herein by reference.
When reference is made herein to a light at a particular wavelength, the reader will understand that reference is being made to light having a spectrum whose peak wavelength is at the particular wavelength.
An optically-pumped vertical cavity surface emitting laser (VCSEL) can be considered to be a type of RSC, and is another example of a semiconductor device designed to emit light. The VCSEL converts at least a portion of a first wavelength light emitted by a pump light source to at least a partially coherent light at a second wavelength. The VCSEL includes a semiconductor multilayer stack that is disposed between first and second mirrors and converts at least a portion of the first wavelength light to the second wavelength light. The semiconductor multilayer stack includes a quantum well that may include a Cd(Mg)ZnSe alloy. Reference is made to pending U.S. Patent Application Ser. No. 61/094,270, “Diode-Pumped Light Source”, filed Sep. 4, 2008, incorporated herein by reference.
Of some interest to the present application are light sources that are capable of emitting white light. In some cases, known white light sources are constructed by combining an electroluminescent device such as a blue-emitting LED with first and second RSC-based luminescent elements. The first luminescent element may, for example, include a green-emitting potential well that converts some of the blue light to green light, and transmits the remainder of the blue light. The second luminescent element may include a potential well that converts some of the green and/or blue light it receives from the first luminescent element into red light, and transmits the remainder of the blue and green light. The resulting red, green, and blue light components combine to allow such a device, which is described (among other embodiments) in WO 2008/109296 (Haase), to provide substantially white light output.
Some devices provide white light using a pixelated arrangement or array. That is, multiple individual light-emitting elements, none of which emit white light by themselves, are arranged in close proximity to each other so as to collectively form a composite white pixel. The pixel typically has a characteristic dimension or size below the resolution limit of the observation system, so that light from the different light-emitting elements is effectively combined in the observation system. A common arrangement for such a device is for three individual light-emitting elements—one emitting red (R) light, one emitting green (G) light, one emitting blue (B) light—to form an “RGB” pixel. Large arrays of the colored light-emitting elements may be addressed in such a way to form a color image. Reference is again made to WO 2008/109296 (Haase), for disclosure of some such devices.
FIG. 1 shows an illustrative device 100 that combines an RSC 108 and an LED 102. The LED has a stack of LED semiconductor layers 104, sometimes referred to as epilayers, on an LED substrate 106. The layers 104 may include p- and n-type junction layers, light emitting layers (typically containing quantum wells), buffer layers, and superstrate layers. The layers 104 may be attached to the LED substrate 106 via an optional bonding layer 116. The LED has an upper surface 112 and a lower surface, and the upper surface is textured to increase extraction of light from the LED compared to the case where the upper surface is flat. Electrodes 118, 120 may be provided on these upper and lower surfaces, as shown. When connected to a suitable power source through these electrodes, the LED emits light at a first wavelength λ1, which may correspond to blue, violet, or ultraviolet (UV) light. Some of this LED light enters the RSC 108 and is absorbed there.
The RSC 108 is attached to the upper surface 112 of the LED via a bonding layer 110. The RSC has upper and lower surfaces 122, 124, with pump light from the LED entering through the lower surface 124. Either one or both of the upper and lower surfaces can be textured to aid in light extraction. The RSC also includes a quantum well structure 114 engineered so that the band gap in portions of the structure is selected so that at least some of the pump light emitted by the LED 102 is absorbed. The charge carriers generated by absorption of the pump light diffuse into other portions of the structure having a smaller band gap, the quantum well layers, where the carriers recombine and generate light at the longer wavelength. This is depicted in FIG. 1 by the re-emitted light at the second wavelength λ2 originating from within the RSC 108 and exiting the RSC to provide output light.
FIG. 2 shows an illustrative semiconductor layer stack 210 comprising an RSC. The stack was grown using molecular beam epitaxy (MBE) on an indium phosphide (InP) wafer. A GaInAs buffer layer was first grown by MBE on the InP substrate to prepare the surface for II-VI growth. The wafer was then moved through an ultra-high vacuum transfer system to another MBE chamber for growth of II-VI epitaxial layers used in the RSC. Details of the as-grown RSC are shown in FIG. 2 and summarized in Table 1. The table lists the thickness, material composition, band gap, and layer description for the different layers associated with the RSC. The RSC included eight CdZnSe quantum wells 230, each having a transition energy of 2.15 eV. Each quantum well 230 was sandwiched between CdMgZnSe absorber layers 232 having a band gap energy of 2.48 eV that could absorb blue light emitted by an LED. The RSC also included various window, buffer, and grading layers.
TABLE 1Band Gap/Refer-Thick-Tran-encenesssition No.Material(nm)(eV)Comment230Cd0.48Zn0.52Se3.12.15quantum well232Cd0.38Mg0.21Zn0.41Se82.48absorber234Cd0.38Mg0.21Zn0.41Se:Cl922.48absorber236Cd0.22Mg0.45Zn0.33Se1002.93window238Cd0.22Mg0.45Zn0.33Se →2502.93-gradingCd0.38Mg0.21Zn0.41Se2.48240Cd0.38Mg0.21Zn0.41Se:Cl462.48absorber242Cd0.38Mg0.21Zn0.41Se →2502.48-gradingCd0.22Mg0.45Zn0.33Se2.93244Cd0.39Zn0.61Se4.42.24II-VI buffer246Ga0.47In0.53As1900.77III-V buffer224InP350,0001.35III-V substrate
Further details of this and other RSC devices can be found in PCT Publication WO 2009/048704 (Kelley et al.).
As demonstrated by the foregoing example, CdMgZnSe alloys are particularly advantageous in the manufacture of RSC devices. Notably, alloys of CdMgZnSe (which for purposes of the present discussion also include CdZnSe) can be made over a wide range of band gap energies and with lattice constants that conveniently allow for their formation as lattice-matched or pseudomorphic multilayer stacks on useful semiconductor substrates. For example, by simply controlling the composition of the CdMgZnSe alloy during the epitaxial growth process, a stack of CdMgZnSe-based semiconductor layers having appropriate thicknesses and band gap energies can be produced on an InP substrate, the semiconductor layer stack containing substantially all of the necessary or desirable components of an RSC so that the resulting RSC, in one embodiment, converts blue pump light to green re-emitted light, and in another embodiment converts blue pump light to red re-emitted light, and in still another embodiment converts blue pump light to both green and red re-emitted light. It is therefore possible to fabricate an RGB pixelated or other white light source by combining a blue-emitting electroluminescent device with one or more RSCs that are entirely or predominantly composed of CdMgZnSe-based materials.